1. Field of the Invention
This invention concerns a sheet guide unit in a sheet-fed press in which the sheet to be printed is fed through a space between the surface of the printing cylinder and a sheet guide unit which directs the sheet along the surface of the cylinder. A stream of air is blown through the space to generate the Bernoulli effect, which causes the sheet to be suspended above the surface as it traverses the space.
2. Description of the Related Art
Multiple-color sheet-fed presses which employ a series of printers each of which prints a different color ink are well known in the prior art. As can be seen in FIG. 9, the basic structural elements of such presses are feeder unit A, which consists of feeder device 39; printer unit B, which has four printers, 132a, 132b, 132c and 132d, arrayed in tandem to print cyan, magenta, yellow and black; and delivery unit C, here paper delivery unit 04.
In multiple-color sheet-fed presses with this configuration, a sucker unit with an inlet for sheets 11, which are piled on table 141 of the feed unit 39, separates a single sheet and transports it on conveyor 120. Swing gripper 121a delivers the sheet to intermediate cylinder 121b of printer 132a. The sheet is fed between blanket cylinder 22a and impression cylinder 23a, and the first color is printed.
Once the first color has been printed, the sheet is fed out between the blanket cylinder 22a and impression cylinder 23a and taken up by intermediate cylinder 27a of the second printer 132b. From the intermediate cylinder 27a, the sheet is delivered to impression cylinder 23b. The next process, the printing of the second color, is executed by blanket cylinder 22b and impression cylinder 23b. 
The subsequent colors are printed one after the other. When sheet 11 is fed out between blanket cylinder 22d and impression cylinder 23d, which perform the final-stage printing, it is pulled onto delivery cylinder 35 of delivery unit C. From delivery cylinder 35, the now completely printed sheet 11 is taken onto chain conveyor 124 and transported to delivery unit 04, where it is added to the stack on table 40 of the unit 04.
Generally, the sheets 11 which are printed in a sheet-fed press are of a thickness which ranges from 0.04 m/m to 0.8 m/m. At times, high-rigidity sheets of metal plate or synthetic resin might also be printed. As the sheet is fed from printer 132a to printer 132b to print the various colors, various mishaps may occur. A thin sheet of paper will generally have low rigidity, and its rear portion will tend to flap. A thicker sheet of paper or sheet metal will have high rigidity, and its reaction force (stability) against the centrifugal force of rotation and its own curvature will cause its rear portion to separate from impression cylinder 23, and collide with the sheet guide unit 1 below the cylinder causing the paper to rebound.
When the paper flaps or rebounds in this way, the print may be smudged or the paper folded or torn. This phenomenon is a significant cause of a reduction in print quality. Two typical methods employed to counteract this problem are to use a skeleton cylinder or a drum cylinder for the intermediate cylinder 27. This allows the most appropriate scheme to be used for the rigidity of whatever sheet is being printed.
The example shown in FIG. 10 is a skeleton-type intermediate cylinder 27, which is used primarily when printing thicker sheets of paper. One of these skeleton cylinders 27 is placed on each side of each printer 132. Each skeleton cylinder consists of a pair of rotors (arms) 271 which rotate on axis 270. Each arm 271 has a series of pawls 29 on its shaft 272 (see FIG. 11 (A)) running from the end of arm 271 to the end of arm 271 on the opposite side of the shaft. The distinguishing feature of the skeleton cylinder 27 is that the area of the cylinder which comes in contact with impression cylinder 23 when the paper passes between them is extremely small. The sheet 100 which is being rotated forward is allowed to bend beyond point P where it comes into contact with pawls 29. In other words, the point of contact P becomes the point of action. By lengthening the distance from this point to the end of sheet 100, we reduce the reactive force exerted by the sheet in its attempt to return to its original shape.
As a result, we reduce the amount of rebounding at the end of the sheet which strikes sheet guide unit 1xe2x80x2, the curved guide which conforms to the hypothetical circumference of the lower portion of skeleton-type intermediate cylinder 27. This scheme minimizes tears and folds; but on the other hand, because this sort of skeleton cylinder 27 provides a larger region in which the end of sheet 100 is free, a thin sheet will have more opportunity to flap.
The feature which distinguishes drum-type intermediate cylinder is that the amount of its surface area which comes in contact with impression cylinder as sheet is fed between them is maximized. Because the portion of sheet which is beyond pawls is guided along the circumference of the drum cylinder, this scheme makes it very difficult for the end of the sheet to flap, so it minimizes doubling, tearing and other defects resulting from the end of the sheet wrinkling or flapping. However, when this sort of drum cylinder is used to convey thicker varieties of paper, the fact that there is very little area where the end of the sheet is free will result in significant rebounding.
In recent years, as print quality has improved, there has been a tendency to use the skeleton cylinders even for thinner papers. To keep thin sheets from flapping, a sheet guide unit 1 is provided which has a sheet guide surface Id following the contour of the lower portion of intermediate cylinder 27 (or 27xe2x80x2) and delivery unit 35 (hereafter referred to as the intermediate cylinder). In order to address the problems in this sort of sheet-fed press, a sheet guide unit is provided in which specifically pressurized air is blown through a number of vents in the sheet guide unit into the space between intermediate cylinder 27 and surface Id of the sheet guide unit. This air is blown along the bottom of sheet 11 as it passes through the space along sheet guide surface id. Because of the Bernoulli effect, the air blown through the vents causes the sheet 11 to be suspended.
One such sheet guide unit is described in Japanese Patent Publication (Kokai) Hei 10-109404. We shall explain the relevant technology with reference to FIG. 10 and FIG. 11. The sheet guide unit, which runs along the circumference of skeleton-type intermediate cylinder 27 or delivery cylinder 35, both of which are studded with pawls 29, consists of air ducts 06. On the upper surface of the air ducts 06 are numerous air vents 4a and 4b. The vents 4a and 4b face in opposite directions and are located on either side of the center of the intermediate cylinder 27 or of delivery cylinder 35. The vents distribute the air toward the outer edges of the intermediate cylinder 27. The vents 4a and 4b produce two streams of air which originate at the vents and continue to move in the directions determined by the vents. These air streams keep the sheet of paper 11 suspended at a specified height, thus stabilizing the travel of the sheet.
In the prior art technique, then, air is blown through a space between sheet guide surface 1d and the intermediate cylinder underneath sheet 11. The sheet is caught on pawls 29 of skeleton-type intermediate cylinder 27, the type of cylinder used for thicker papers. The air is blown into the space from ducts 06 below the guide surface through the air vents 4a and 4b. The Bernoulli effect which results from the differential flow rate above and below the sheet causes the sheet 11 being conveyed around the circumference of the intermediate cylinder 27 to be pulled toward surface 1d of the sheet guide unit and to be suspended slightly above that surface as it is conveyed until it is delivered to the subsequent impression cylinder 23.
However, in this prior art technique, when the sheet exits the guide space and is released from the pawls of the skeleton cylinder, there is nothing to hold it. And particularly if the sheet is thin, the Bernoulli effect due to the flow velocity of the air stream will not be sufficient to stabilize the end of the sheet.
When a sheet of a thinner stock is to be conveyed along a skeleton-type intermediate cylinder 27, the end of the sheet will inevitably flap or flutter in the downstream portion of the sheet guide. This may result in a variety of imperfections, including wrong positioning, overprinting, or crumpled or torn paper.
In view of these problems in the prior art, the object of this invention is to provide a sheet guide unit which allows sheets of thinner stocks to be conveyed in a stable fashion, and allows these sheets to be conveyed smoothly even when a skeleton cylinder, which is better suited to thicker stocks, is used. The sheet guide unit for a sheet-fed press according to this invention has a sheet guide space between the printing cylinder and a sheet guide unit, and it would prevent specially the end of the sheet from flapping or fluttering in the downstream portion of its travel through the sheet guide space.
To solve this problem, this invention will disclose the sheet guide unit provided for a sheet-fed press which has a first printing cylinder, such as an intermediate or delivery cylinder below which is fashioned a curved guide surface separated by a small space from the surface of the cylinder; and a second printing cylinder, such as an impression cylinder or the like which is positioned quite close to the first cylinder so that the reception unit for the sheet is between the two cylinders. The sheet guide unit according to this invention is distinguished by the following features. It has one or more air supply chambers, which are behind the curved sheet guide surface in the upstream portion of the path traveled by the sheet; and an aspiration chamber behind the curved sheet guide surface adjacent to the air supply chamber in the downstream portion of the sheet traveling path; a first air control means to control the supply air, which is blown from the air supply chamber through air vents provided in the upstream portion of the path, and conveys the sheet through the sheet guide space suspending over the downstream portion of the path; a second air control means to control the aspiration air, which is drawn into the aspiration chamber via the numerous first aspiration vents in the downstream portion of the path, and exhausted from the aspiration port provided on one of the walls of the aspiration chamber which is not the sheet guide surface.
With this invention, different means are used to control the air stream on the upstream and downstream portions of the sheet guide surface as the sheet is conveyed. This scheme insures that a sheet of a thinner stock will be conveyed smoothly even when the press uses a skeleton cylinder.
In the upstream portion of the sheet traveling path through the sheet guide space over the sheet guide surface, as can be seen in prior art designs, a difference in the flow velocity of the air stream above and below the sheet cause the Bernoulli effect to occur. This causes the sheet being conveyed along the surface of the intermediate cylinder to be drawn toward the surface of the sheet guide and suspended slightly above it, thus enabling the sheet to be conveyed smoothly without flapping.
In the downstream portion of the path traveled by the sheet through the guide space over the sheet guide surface, there is nothing mechanical to hold the sheet once it is released by the pawls of the skeleton-type intermediate cylinder. It is instead held by the negative pressure created by the set of first aspiration vents which extend across the entire width of the sheet guide surface, and which communicate with the aspiration chamber. Once the sheet is released from the grip of the pawls, it is drawn toward the surface of the sheet guide unit in the downstream portion of the path it travels through the guide space, which has numerous aspiration vents running across it.
Thus even when a sheet of a thinner stock is being printed using a skeleton cylinder, when the end of the sheet is released by the pawls of the skeleton cylinder in the downstream portion of the sheet""s path, it will always be drawn toward the surface of the sheet guide by the negative pressure generated by the suction of the first aspiration vents. This will prevent the end of the sheet from flapping or buckling.
In both the upstream and downstream portions of the sheet guide unit, then, the sheet will be transferred smoothly from the intermediate cylinder in question to the next printing cylinder. Sheets of thinner stocks will be conveyed in a stable fashion. Thinner sheets, then, can be printed smoothly even when a skeleton cylinder, which is more suitable for thicker stocks, is used, and printing defects can be prevented.
Another embodiment of this invention comprises a sheet guide in a sheet-fed press which has on the outlet end of the sheet guide unit a series of air passages consisting of cut-away portions through which the pawls of the printing cylinder can pass. In the downstream portion of the sheet guide unit mentioned above, a space (hereafter referred to as xe2x80x9caspiration spacexe2x80x9d) is provided which faces the surface of the second printing cylinder. The air which flows through the air passages along with the air drawn into the aspiration chamber through the first aspiration vents is drawn into the aspiration chamber, and then exhausted via the aspiration port.
Even though it is possible for turbulence to occur in the air stream flowing through the sheet guide space and the intervals provided on the outlet end of the sheet guide unit to accommodate the pawls of the printing cylinder, this air is constantly sucked into the aspiration chamber before the turbulence can reach a significant level. This arrangement prevents the end of the sheet from flapping or buckling, and the sheet is transferred to the next printing cylinder without hindrance.
Another feature of this invention is that an air volume adjustment means is provided by which the volume of air drawn into the aspiration chamber via the first array of vents in the downstream portion of the sheet guide can be controlled. By this means, a portion of the air exhausted from the aspiration chamber can be returned to the same chamber. This insures that the suction provided will never be sufficient to impede the passage of the sheet through the downstream segment of the sheet guide unit, but will be sufficient to keep the end of the sheet from flapping or buckling so that it can be conveyed smoothly.
The aspiration port of the second air control means comprises a recirculation path which connects the aspiration chamber and the air supply chamber. The air may be recirculated along this path with the help of a recirculation pump installed on the path.
With this invention, the air which is continuously recirculated through the recirculation channel is also the air which flows through the sheet guide space. This scheme insures a smooth flow of air and makes it more unlikely that eddies will form. The sheet moves through the guide space in a stable fashion, and the air recirculation pump can be used to move the air along both the main and the recirculation paths, thus reducing the equipment cost.
As was discussed previously, if the system is configured in such a way that a portion of the air driven by the recirculation pump can be returned to the aspiration chamber, it will be possible to adjust the volume of air sucked into the aspiration chamber through the first aspiration vents in the downstream portion of the sheet guide unit.
The first aspiration vents on the sheet guide surface should be divided into two subsets by an imaginary line drawn from side to side through the midpoint of the sheet guide. As they proceed to the sides of the sheet, these rows of vents should all shift slightly upstream or downstream with respect to the path of the sheet such that the phase of each row is shifted slightly from that of the previous row.
For example, the aspiration vents in the very center of the guide might be shifted slightly upstream from the vents on the lateral sides of the guide. Then the center of the tail end of the sheet moving through the sheet guide space will leave the vents before the sides do. Since the sides of the sheet leave the vents last, this scheme is well suited for use with thinner stocks of paper, as they are liable to experience flapping and buckling on the sides of the sheet.
If the aspiration vents on the sides of the guide are shifted slightly upstream from the vents in the center, the sides of the end of the sheet moving through the space will leave the vents before the center does. Since the center of the sheet leaves the vents last, this scheme is well suited for use with thicker stocks of paper, as they are liable to experience flapping and buckling in the middle of the sheet.
Another feature of this invention is that the aspiration chamber is divided into several chambers by partitions at intervals along the width of the sheet guide. Then the ability to alter the volume of the air aspirated into each of the sub-chambers or cut it off completely constitutes a control means to control the volume of air aspirated.
With this invention, the valves which constitute the control means can be adjusted to change the pressure (i.e., the negative pressure) along the width of each chamber. This, in effect, adjusts the suction along the width of the sheet guide, allowing the position of the sheet to be controlled along its width. This insures that the sheet will maintain the same position and will not shift toward one side or the other as it travels.
Yet another embodiment of this invention for the sheet guide unit has a series of openings at intervals through which air can pass provided at the outlet end of the sheet guide surface as disclosed in claim 2. The embodiment is distinguished by the fact that a second aspiration vents, into which the air flowing through the openings is sucked, is provided on the wall of the aspiration chamber facing the surface of the second printing cylinder. This air, along with the air sucked into the aspiration chamber via the first aspiration vents, is exhausted via the aspiration port.
With this invention, the air in the aspiration space near the outlet end of the sheet guide unit is sucked into the aspiration chamber through the second aspiration vents. This creates an air stream in the aspiration space with a velocity component in the direction of the second aspiration vents. The resulting Bernoulli effect generates a negative pressure in the aspiration space.
This negative pressure pulls the end of the sheet toward the surface of the downstream portion of the sheet guide unit. Even at the very end of the sheet guide space, then, the end of the sheet is prevented from flapping or buckling.
Another preferred embodiment of this invention is a sheet guide unit for a sheet-fed press in which the outlet end of the sheet guide surface has a series of openings at intervals through which air can pass as disclosed in claim 2. The invention here implemented is distinguished by the following. In this sheet guide unit, a hood is provided over the rotary surface of the second printing cylinder, which is below the end of the sheet guide unit. The hood is adjacent to the wall in the downstream portion of the aspiration chamber which faces the surface of the second printing cylinder. The third aspiration vent which communicates with the aspiration chamber and the hood is provided in the downstream wall of the aspiration chamber. On the bottom of the hood an aspiration vent is provided. The air which enters the sheet guide space via the air passages at the end of the guide as well as the air sucked in the hood from the aspiration chamber through the third aspiration vents is sucked out via the hood.
With this invention, the air in the vicinity of the reception unit is collected and sucked into the hood. This prevents the air from dispersing and exerting an undesired influence on the passage of the sheet. The sheet can be handed of f from one printing cylinder to the next all the more smoothly.
As was mentioned previously, the system should be designed so that the air which ends up in the hood can be recirculated by means of a pump installed on the aspiration and recirculation path to both the supply chamber and the aspiration chamber.